1. Field of the Invention
The present invention relates, in general, to a method for manufacturing an electrode for phosphate type fuel cells and, more particularly, to the use of a combination of a coating process and a rolling process in manufacturing an electrode, which exhibits the advantages of few cracks in an electrocatalyst layer and strong adhesiveness between an electrode support and the electrocatalyst layer in a phosphate type fuel cell.
2. Description of the Prior Art
Generally, fuel cells produce clear electricity with high efficiency, in which the hydrogen contained in hydrocarbon fuels, such as methanol or natural gas, and the oxygen in the air are directly converted to electric energy through an electrochemical reaction. Fuel cells were first developed as a power supply for space crafts in U.S.A. in the 1970's, since when dynamic research and development have been directed to applying the fuel cells for general use. Currently, developed countries including U.S.A. and Japan are actively making an effort to put them to practical use (see, J. Appleby, "Assessment of research needs for advanced fuel cell", Energy Inter. J. vol.11. no. 1/2. pp. 13-94, 1986).
Fuel cells are largely classified into three groups: phosphate type; molten carbonate type; and solid electrolyte type. Of them, the phosphate type fuel cell is the most advanced in the research and development for practical use.
The main body of a phosphate type fuel cell consists of unit cells, each comprising a phosphate-impregnated electrolyte layer to the opposite sides of which a cathode and an anode are attached. A fuel cell electricity generation system with a capacity of several hundred kilowatts to several megawatts can be built up by stacking such unit cells in a multi-layer manner.
The capability of a phosphate type fuel cell is dependent on the electrochemical properties of its elements. Of them, especially, electrodes are known to determine most of the capability of the fuel cell.
Having a great influence upon a fuel cell's capability of producing electricity, an electrode of a phosphate type fuel cell consists of an electrode support and an electrocatalyst layer. The electrode support is formed of porous carbon paper and the electrocatalyst layer is a porous gas diffusion electrocatalyst layer in which carbon particles coated with fine platinum powder are joined with each other by polytetrafluoroethylene (PTFE) particles.
While the electrode support plays a role of supplying the reactant gases such as hydrogen and oxygen to the electrocatalyst layer, the electrochemical reaction of the fuel cell occurs in the electrocatalyst layer.
There are well-known two processes for manufacturing a phosphate type fuel cell; coating process and rolling process. In order to better understand the background of the invention, a description will be given of the processes, below.
First, according to a coating process, as disclosed in Japanese Pat. Laid-Open Publication No. Heisei 7-22035,an electrode is obtained by forming an electrode support and an electrocatalyst layer slurry on the electrode support and coating the catalyst layer. The manufacture of an electrode is completed by drying and sintering the electrode.
This conventional coating process of manufacturing an electrode of a phosphate type fuel cell has a major disadvantage that an abundance of cracks occur in the catalyst layer during the drying and sintering. An appropriate number of cracks formed in the catalyst layer may improve the performance of a fuel cell but a large number of cracks in the catalyst layer cause phosphate electrolyte to flood into the catalyst, greatly deteriorating the performance of the fuel cell.
It is preferable to prevent the occurrence of cracks in the catalyst layer as to a maximal extent as possible but the coating process has a great difficulty in restraining an abundance of cracks.
Further, another significant problem of the conventional coating process is that, because the force with which the carbon particles within the electrocatalyst layer attract themselves and the binding strength between the electrode support and the electrocatalyst layer are very weak, the electrocatalyst layer readily separates from the electrode support.
U.S. Pat. No. 4,603,060 discloses a rolling process of manufacturing an electrode of a phosphate type fuel cell. First, a modification is done to make a gum-like catalyst layer slurry, which is formed into a sheet type catalyst layer by use of a roll. This sheet type catalyst layer is attached to an electrode support by using a press, to give the electrode.
As mentioned, the gum-like catalyst layer is made sheet-like in advance but it is difficult to make the catalyst layer wide and uniform in thickness. In addition, a press with a wide area is required to attach the wide catalyst layer to the electrode support.
Particularly, when pressing the electrocatalyst layer upon the electrode support, a uniform stress must be applied over all the surface of the catalyst layer. If there is an application of non-uniform stress, the catalyst layer is, in part, delaminated. In practice, the manufacture of a press capable of applying wide and uniform stress is accompanied by great economical and technical difficulty.